The telecommunications industry keeps witnessing rapid changes in the way people and organizations communicate. Many of these changes spring from the explosive growth of the Internet and from applications based on the Internet Protocol (IP). The Internet has become an omnipresent means of communication, and the total amount of packet-based network traffic has quickly surpassed traditional voice (circuit-switched) network traffic, such as PSTN's (Public Switched Telephony Networks) and the like.
Technological advancements have helped telecommunication service providers, users, and suppliers realize that voice traffic and services may be one of the next major inroads to take full advantage of IP. This expectation is based on the impact of a new set of technologies generally referred to as Voice over IP (VoIP) or IP telephony.
VoIP supplies many unique capabilities to service providers and users who depend on IP or other packet-based networks. The most important benefits include the following: cost savings, open standards and multi-vendor interoperability, and integrated voice and data networks.
In the case of cost savings, users can reduce or eliminate toll charges associated with transporting calls over the PSTN by moving voice traffic to IP networks. In the case for open standards and multi-supplier interoperability, both service providers and/or users can purchase equipment from multiple suppliers and eliminate their dependency on proprietary solutions. Finally, in the case for integrated voice and data networks, service providers and/or users can build truly integrated networks for voice and data by making voice “just another IP application”. These integrated networks not only provide the quality and reliability of today's PSTN, but they also enable users to quickly and flexibly take advantage of new opportunities within the changing world of communications.
The first VoIP products were targeted at users looking to reduce telecommunication expenses by moving voice traffic to packet networks. To take advantage of favorable regulatory treatment of IP traffic and without any established standards, most early implementations were based on proprietary technology.
Session Initiation Protocol (SIP) was designed as a multimedia protocol that could take advantage of the architecture and messages found in popular Internet applications. By using a distributed architecture—with universal resource locators (URLs) for naming and text-based messaging—SIP attempts to take advantage of the Internet model for building VoIP networks and applications. In addition to VoIP, SIP may be used for video-conferencing and instant messaging. As a protocol, SIP defines how sessions are to be set up and torn down. SIP may be used to invite a user to take part in a point-to-point or a uni-cast session. It utilizes other IETF protocols to define other aspects of VoIP and multimedia sessions, such as Session Description Protocol (SDP) for capabilities exchange, URLs for addressing, Domain Name Systems (DNS's) for service location, and Telephony Routing Over IP (TRIP) for call routing.